Bob Seely is a member of the Foreign Affairs Select Committee, and is MP for the Isle of Wight.

The case of Shamima Begum, who ran away to live under ISIS rule when she was a teenager, is deeply troubling. In 2015, aged just 15, she went to Syria to support the terror group, and was almost immediately married to a Dutch jihadi convert. She now wants to return to the UK with her surviving child. Two other are dead.

She is one of hundreds of former and current ISIS supporters who hold UK passports, and who now may try to make their way back to Britain as ISIS faces final collapse.

Before I entered Parliament, I served with our armed forces during the campaign to destroy ISIS’ so-called caliphate. I was proud to do so. The territory that ISIS controlled, which initially stretched from central Syria through to Mosul in Northern Iraq, was a true heart of darkness. It was a revolting regime that mixed mediaeval theocracy with police state practises, and which advertised its death cult in infamous beheading videos.

Four years of bombing and ground force assault by the US, its British and French allies, our Kurdish partners on the ground in Iraq (the Peshmerga) and Syria (the SDF) have defeated ISIS as a physical force, but this victory intensifies a problem: what are we to do with returning ISIS fighters and their fellow travellers? What do we do with those who continue to nurture the idea of violent jihad in their minds? Getting our decision wrong could cost lives.

There is a natural – and exceptionally understandable – instinct to feel anger and contempt for the decisions made by Begum and others. The public revulsion has been rightly expressed by Sajid Javid.

However, it has proved hard to prosecute those who went to live in the ISIS-controlled area. As a result, Javid and his team steered through the Counter Terror and Border Security Act, which this week became law. First, it brings in a designated area offence, allowing prosecution for being in a geographical location without good reason. Second, it makes revoking UK citizenship easier. Third, it brings in Terrorism Prevention and Investigation Measures – ‘super ASBOs’ – to disrupt those engaged in extremism in the UK.

However, this law can’t be used retrospectively. In addition, if Begum is a British citizen and does not have a second citizenship, she has the right to return. This is not a negotiable point. It is illegal to make her stateless, and attempts to do so will see the Government in court. Furthermore, she was a child when she left. She has made some dire life choices, but her age should be taken into account. Either way, if she makes it to our shores, we will have to find a solution for her and for people who have done worse.

Public anger is understandable, but our priority must be public safety – and that means making some difficult choices.

In practical terms, it means continuing to develop intelligence on ISIS returnees. We need to be ‘collecting’ on both UK and other ‘internationals’ who served ISIS. We need to do so to be able to make judgements on their relative danger to our societies, how we monitor them and how they can be deradicalised. The more information we have, the more we can judge which returnees are a threat. Everything we do, including the deals we strike and whom we decide to prosecute, has to be based on that.

Back in 2016, it was reckoned that 700 UK citizens were fighting for, or supporting, ISIS. That figure now totals between 800 and 1,000. Of those, between 100 and 250 have died. UK air power killed some of them; the US and the French others. More were killed by our Kurdish the Peshmerga and the SDF. Other UK fighters who survived and who have a second passport will not be able to return – because they have been quietly stripped of their UK citizenship.

However, even if we identify most of those British citizens who served ISIS and are now considering returning, we will miss some of them. However good our agencies’ information is, some will have slipped through. Therefore, the need for information, on both known and unknown ISIS terrorists and fellow travellers, is our priority. The greatest protection we have against another Manchester bombing, 7/7 or Borough attack is knowledge.

We do not have to help ISIS terrorists and their war brides to return. But for those who make it here, whether they are prosecuted or not, there must be a price for returning and living their lives in the freedom that they denied others when they lived in ISIS-controlled territory. That price is information.